The prior art LED light source device for a pulse encoder is illustrated in FIGS. 1, 2 and 3. A rotary code plate 1 for pulse encoder is schematically illustrated in FIG. 2. Among the codes of phases A, B and Z, the code of the phase A will now be explained. A sequence of light interrupting patterns "a" are formed by means of metal vapor deposition on the surface of the code plate 1, with a predetermined interval between the adjacent patterns. A sequence of light interrupting patterns "f" are formed by means of metal vapor deposition on a fixed plate 3 located beneath the sequence of the light interrupting patterns "a". The sequence of the light interrupting patterns "f" corresponds to the sequence of the light interrupting patterns "a".
LEDs 21, 22 and 23 are arranged against the patterns of the phases A, B and Z on the pattern plate 1 as illustrated in FIG. 1, at the positions 21, 22 and 23 illustrated in FIG. 2 by broken line circles.
In the phase A, for example, the light emitted by the LED 21 passes through a light transmitting portion between the light interrupting patterns "a" on the pattern plate 1, then, passes through a light transmitting portion between the light interrupting portions f on the fixed plate 3 and, finally, is received by photo cells 41 and 42. In the arrangements illustrated in FIG. 2, the light emitted by the LED at the position 21 above the pattern plate 1 and forwarded to the left-side portion of the pattern plate 1 is directed to the photo cell 41, while the light emitted by the LED at the position 21 above the pattern plate 1 and forwarded to the right-side portion of the pattern plate 1 is directed to the photo cell 42. FIG. 2 shows the status in which the above mentioned left-side light is able to pass through the pattern plate 1 and the fixed plate 3 because the light transmitting portion between the light interrupting pattern "a" of the pattern plate 1 coincides with the light transmitting portion between the light interrupting pattern "f" of the fixed plate 3; while the above mentioned right-side light is unable to pass through the fixed plate 3 because the light interrupting pattern "f" of the fixed plate 3 coincides with the light transmitting portion between the light interrupting pattern "a" of the pattern plate 1.
Accordingly, when the light signals are converted into electric signals by the photo cells 41 and 42, the right-side photo cell 42 produces an electric signal P which has a phase difference of 180.degree. with regard to an electric signal P produced by the left-side photo cell 41. These electric signals P and P are applied to a differential amplifier which produces a desired output. In this differential amplification system, it is required that the amount of the left-side portion of the light emitted by the LEDs 21, 22 and 23, and directed to the photo cell 41, be the same as the amount of the right-side portion of the light emitted by the LEDs 21, 22 and 23, and directed to the photo cell 42.
The prior art circuit for supplying electric power to the LEDs is illustrated in FIG. 3. The LED 21, the LED 22 and a resistor R.sub.1, connected in series in a branch k, are connected in parallel with the LED 23 and a resistor R.sub.2, connected in series in a branch l. In this circuit, when a source voltage +E varies, the rates of variations of the currents of the LED 21 and the LED 22 are different from the rate of variation of the current of the LED 23, and accordingly, the rates of variations of the lights of the LED 21 and the LED 22 are different from the rate of variation of the light of the LED 23. The reason for these differences can be explained as follows. It is assumed that the current of the branch k is i.sub.1, the current of the branch l is i.sub.2, the voltage drops across the LED 21, LED 22 and LED 23 are e, respectively, the current reductions of the LED 21 and LED 22 when the source voltage +E drops .DELTA.E are .DELTA.i.sub.1, respectively, and the current reduction of the LED 23 when the source voltage +E drops .DELTA.E is .DELTA.i.sub.2. The rates of variations of the currents i.sub.1 and i.sub.2 of the branches k and l are calculated, respectively, as follows. ##EQU1##
Therefore, the rates of variations of the currents i.sub.1 and i.sub.2 of the branches k and l are different. Under the unavoidable condition that the amount of the left-side portion of the light emitted by the LEDs 21, 22 and 23 and directed to the photo cell 41 is different from the amount of the right-side portion of the light emitted by the LEDs 21, 22 and 23 and directed to the photo cell 42, the difference of the rate of variation of the light between the LEDs 21 and 22 and the LED 23 exerts an undesirable influence on the output signal of this pulse encoder obtained by means of the differential amplification of the outputs P and P of the photocells 41 and 42.